Plasma enhanced etch processes are employed for etching thin film structures on semiconductor wafers. One plasma enhanced etch process for etching a dielectric thin film, such as silicon dioxide, employs fluorocarbon and fluoro-hydrocarbon process gases. Such process gases dissociate into fluorine-rich etchant species and carbon-rich polymer species. The etchant species perform the desired etching while the polymer species provide desired passivation, as is well-known in the art. The polymer species tend to deposit on both the wafer front side as well as on the exposed edge portion of the wafer backside. Polymer deposition on the wafer backside may occur in some cases because the edge periphery of the wafer may be exposed during the etch process. Polymer deposited on the wafer front side is removed in the normal course of subsequent plasma process steps. However, the backside polymer film is not removed, at least in part because the wafer backside is shielded from plasma ion bombardment. Such backside polymer film builds up uncontrollably and can contribute to contamination during later plasma processing steps. We now propose to prevent such contamination by removing the backside polymer at the conclusion of the plasma etch process. The problem is how to accomplish this.
A special etch process could be performed to remove the backside polymer. The problem is that the wafer front side has certain delicate thin film materials, some of which may have been exposed during the prior dielectric etch step. The delicate thin film may be an ultra-low dielectric constant (ULK) material, for example. Such ULK materials tend to comprise a relatively high proportion of carbon (as does the backside polymer), and are therefore particularly susceptible to etching or damage when exposed to plasma chemistries capable of removing the backside polymer. It is very difficult or impossible to prevent damage to a ULK film on wafer front side while exposing the wafer to an environment capable of removing backside polymer.